What Is A Floor Joist?

A floor joist is a key component in home construction. Joists are building materials that stretch horizontally between a building foundation, walls, and structural beams. The main function of floor joist installation to support a ceiling or floor’s weight transfer in a structure.

With floor joists, there isn’t much you need to know. However, it doesn’t hurt to be knowledgeable on the finer details of your home. You should be aware of the different materials that make up your floor system and how laminated veneer lumber and old growth trees are used. 

What Are Floor Joists?

What Is A Floor Joist?

A floor joist is a horizontal beam, usually made of wood, that is used to support and enclose the flooring above a foundation. Floor joists transfer loads to the vertical beams of the house.

Most floor joists are single or double, and most often made with 2x6s or 2x8s. Though on occasion, they are made of 2x10s and tripled for additional support. But minimum floor joist sizes are never less than 2×6.

Floor joist installation isn’t always necessary for a house because many floors are built directly onto a concrete slab. For homes built over basements or on wood foundations, floor joists are required for structural integrity. 

Types Of Floor Joists

Types Of Floor Joists

There are three types of floor joists used by contractors in wooden structures. 

Solid Lumber

The wood species used for floor joists include Douglas fir. Solid lumber is the cheapest type of floor joist if you use standard wood, rather than specialty wood. But in general, solid lumber is more fire-resistant and water-resistant than other options.

Because of its price and a large array of options, solid lumber is the most popular type of floor joist. It’s easy to use, understand, and is the simplest type of floor joist. If you know anything about construction, then you can’t mess it up. 


I-beams look like the letter I. The top and bottoms of the I are made with either solid wood or laminated veneer, but the center is made with plywood or OSB. They can span a longer distance than solid lumber.

Because they are made with plywood, they can be cheaper than other options and can be stacked and glued to create a stronger board than what one single layer of plywood might be. This makes I-joists very strong. 

Floor Trusses

Floor trusses are made with 2×4 wood planks on the top and bottom with a “web” of diagonal boards in the middle. The boards are secured with metal plates in the floor trusses. Because of their design, they use less wood than normal.

Floor trusses also have gaps that leave room to make wiring and plumbing much easier. However, crawlspaces are tricker with web floors because there isn’t enough room to crawl through like there would be with solid wood. 

Load-Bearing Floor Joists

Load-Bearing Floor Joists

Floor joists are designed to be load-bearing. They act as the support for upper levels, often transferring weight to vertical supports within a foundation. With upper levels, floor joists support the walls. 

Supporting Structures

Walls support the next floor. If a wall is placed parallel to the joists, it isn’t load-bearing. Only walls placed perpendicular to the floor joists can be load-bearing. For more info on load-bearing walls, check this article out

Supportive Framework

What you need to know is floor joists bear the weight of the entire home. While walls usually only bear part of the weight, with some not bearing any weight at all. Floor joists are important for this reason alone. 

Load Capacity

You will need to determine the load capacity of your floor trusses. In residential home floors the live load is considered to be 30 to 40 pounds per square foot (psf), although this varies depending on the location inside your home. First-floor live loads have higher requirements than second-floor live loads 

Floor Joists Building Codes

Floor Joists Building Codes

Local building codes requires wood joists to support at least 10 pounds per square foot for a dead load and 30 pounds per square foot for a live load. Different wood, grades, span length, and so on affect the support per square foot. 

At this point, you’re probably wondering what the difference between a live load and a dead load. 

A dead load is a permanent load. It comes from the structure and load of a building. But then live loads are temporary loads. They are applied over time, including traffic, furniture, and more. 

Repairing Floor Joists

Floor joist repairs should be handled by a professional. For simple repairs to floor joists, consider using a flitch plate. A flitch plate is a thin piece of steel or plywood that is bolted to a weak joist to offer support. However, another option is to use an identical board and secure it to the old one.

You can leave the older board as the new board is there for additional support and will become the new load-bearing board in the area. 

Floor Joist Spacing

Floor Joist Spacing

Joist spacing is the most important part of floor joist installation. If the joists are spaced too far from each other, they won’t work. On the other hand, if the floor joists are too close to each other, then it means you’ve spent too much money.

There are several factors affect floor joist spacing. The first thing is lumber grade, which can change the outcome by an inch or two. Board size also changes everything.

While the spacing codes have changed over the years, this is where we stand today. Here are the three most common spacings with floor joists and covers 90 percent of all types of spacings for floor joists. 

  • 2×6 – 12 inches
  • 2×8 – 16 inches
  • 2×10 – 24-inches

Wood joists should never be spaced more than 24-inches apart. So both 2x12s and 2x14s need to be spaced no further apart than 24-inches just like the 2x10s. 

Shorter widths should be further apart. But this is an estimate depending on grade and selected integrity. The vertical boards and lengths of the horizontal boards are also important. 

How To Choose The Best Boards For Floor Joists

Picking The Best Boards For Floor Joists

Choosing boards for floor joists for an open web floor is can be difficult considering their importance. You need to know the differences between high bending strength and medium bending strength, for example. 

Wood Type

Different wood species have unique characteristics. Some are more flexible while others are stronger. Most of the time, slow growing trees, like the Douglas fir, are stronger. While fast-growing trees are soft wood.

Stronger trees with good elasticity are ideal for floor joists, like the Ponderosa pine. Some popular choices include pine, fir, and spruce. These hardwoods, which you’ll notice are conifers, are great options and are easy to find.

Wood Grade

Wood grade is important because lower grades indicate more defects, so it’s not structurally strong. So, the lower the number, the better the wood will be. With lumber graded wood, higher grades have fewer defects. Meanwhile, lumber with lower grades have a shorter life span. 

Using Grade-1 isn’t necessary as Grade-2 is flawless for most projects, including floor joist installation. You should avoid using Grade-3 or lower as it will risk the integrity of your foundation. 

Western red cedar, and Eastern white pine have low bending strength.

Wood Size

When it comes to the size of the boards, avoid using anything smaller than a 2 x 6. The largest you will use is a 2 x 12, but most common sizes will be 2 x 8 or 2 x 10. Though 2 x 6s are a common choice, they’re not your cheapest option.

If you choose 2x6s, you have to space them closer together, which would be less than 16 inches while you can space 2 x 12 24-inches apart. 

Frequently Asked Questions (FAQ)FAQ

Can I Install Floor Joists?

Unless you’re a structural engineer or professional contractor, it wouldn’t be a good idea for you engage in floor joist installation. You can paint your walls and install non load-bearing walls, but where structural integrity is concerned, floor joist installation is not a DIY project. 

The same goes for electricity and water. You wouldn’t put in your own septic. So keep a set list of things that you hire a contractor for. In the end, it will save you money, time, and a whole lot of restless nights. 

Should Floor Joists Run The Same Direction As Roof Rafters?

In residential floors, floor joist span will run the shorter width of a room. With ceilings that have rafters, they will run parallel to the rafters. 

How Do You Know Which Way A Floor Joist Goes?

When you hear a squeak, that’s where the joist is. Joists typically run in the same direction throughout your home. Check your attic and see which way they run and chances are it will be the same anywhere else in your home.

What Happens If A Floor Joist Breaks?

You can tell when you have a broken floor joist when you floor bounces or sags. You’ll also hear loud squeaking sounds.

How Much Does It Cost To Repair Floor Joists?

Basic floor joist repairs on floorboards will cost between $500 to $700 for a small area of flooring. Floor joist repair, in general, costs between $6,000 to $10,000 for a small room.

Do Termites Eat Floor Joists?

Yes, termites eat floor joists. Termites eat wood, and this includes floor joists. You’ll know termites have attacked your floor joists when your floor structure begins to sag or your floorboards start rising. 

Floor Joist Conclusion

The floor joist plays a major role in residential construction. For a structural engineer, working with floor joists sizes is part of their job, yet a DIY enthusiast wouldn’t have a clue how to use them. When working with joists, there are many things to consider. You’ll need to know if you’re working with a bearing wall or foundation wall, as an example.

A joist spans a ceiling and floor spaces in residential homes. Your floor at home has a load capacity and uses one of the many types of floor joists to transfer weight. Local building codes mandate that homes use floor joists. With floors or ceilings that have longer spans, heavier joists are required. 

On occasion, floor joists run perpendicular to rafters, when they typically run perpendicular. You should know that installing floor joists is not a DIY project for your home. And this applies to all DIYers. If you have problem with your floor joists, call a professional contractor. If you tamper with the joists in your home, you could be setting yourself up for major problems down the road.